1. Field of the Invention
The present invention relates to a waveguide-type optical matrix switch used in optical fiber communications, and more particularly, to a waveguide-type optical matrix switch which is little affected by fabrication errors, and has a high extinction ratio.
2. Description of the Prior Art
Recently, it has become essential for further spread of the optical fiber communications to develop optical circuit components such as optical splitters and couplers, optical multi/demultiplexers and optical switches in addition to the realization of higher performance and lower cost optical fibers, photo-detectors and emitters. Above all, optical switches are considered to play an important role in near future to freely switch optical fiber lines in response to demands or to establish alternate routes in case of line faults.
As typical optical switch configurations, bulk-type and waveguide-type optical switches have been proposed. The bulk type optical switch is arranged by employing a movable prism and lenses as its parts, and has advantages that it has small wavelength dependence, and relatively low loss characteristics. The bulk-type optical switches, however, have not much spread because they are not suitable for mass production since their assembly and adjusting processes are complicated and expensive. On the other hand, the waveguide-type optical switches, which are mass-produced in the form of so-called integrated optical switches by utilizing the photolithography and fine pattern fabrication technique, are considered as a future-type optical switch. In particular, the waveguide-type optical switch is considered essential to realize a practical, rather large-scale M.times.N optical matrix switch having M input ports and N output ports.
FIG. 1 is a schematic diagram showing an arrangement of a 4.times.4 optical switch as an example of an M.times.N optical matrix switch that will become the subject matter of the present invention. The 4.times.4 matrix switch are arranged in such a fashion that four input optical waveguides 1a, 1b, 1c and 1d intersect four output optical waveguides 2a, 2b, 2c and 2d at 16 places. Each of these 16 places is provided with a 2.times.2 optical switch elements S00, . . . , or S33 as a minimum unit optical switch. Such an arrangement of the optical matrix switch is called "a strictly non-blocking optical matrix switch", and can switch four-channel light signals entering the input optical waveguides 1a, 1b, 1c and 1d to any one of the four output optical waveguides 2a, 2b, 2c and 2d.
For example, when a light signal incident to the input optical waveguide 1a is to be outputted from the output optical waveguide 2b, an optical path passing through the optical switch elements S03, S02, S01, S11, S21 and S31 is formed. In this case, in the optical switch element S01, a bar path is established which guides a light beam incident to the bottom left waveguide to the bottom right waveguide. In the other switch elements, a cross path is established which guides a light beam incident to the bottom left waveguide (or to the top left waveguide) to the top right waveguide (or to the bottom right waveguide). To minimize the number of driven switch elements, it is necessary to establish a cross path when an optical switch element is in the OFF state, and a bar path when an optical switch element is in the ON state. In the above-mentioned example, only the switch element S01 is made ON state, and the other switch elements are made OFF state. This holds true for any other optical paths. For example, an optical path from the input optical waveguide 1a to the output optical waveguide 2a can be established by making the optical switch element S00 ON, and the other optical switch elements S03, S02, S01, S10, S20 and S30 OFF. Thus, the number of the optical switch element to be made ON to form a bar path is always one, whereas that of the optical switch elements to be made OFF to form a cross path varies from zero to six. In other words, in the 4.times.4 optical matrix switch, the number of the optical switch elements through which the light signal passes varies from a minimum of one to a maximum of seven.
Many attempts have been conducted to constitute the optical matrix switch by using optical waveguides of various kinds of materials. Above all, a thermooptic matrix switch utilizing thermooptic effect of silica-based optical waveguide on a silicon substrate is expected as the most promising candidate of the practical optical matrix switches because it has no unfavorable polarization dependence, and has good joining characteristics to optical fibers.
FIG. 2A is a plan view showing the entire arrangement of a conventional thermooptic 4.times.4 optical matrix switch constructed on a silicon substrate as an example corresponding to the 4.times.4 optical matrix switch as shown in FIG. 1, and FIG. 2B is an enlarged plan view showing an arrangement of a conventional optical switch element of FIG. 2A. In these figures, eight optical waveguides including the four input optical waveguides 1a, 1b, 1c and 1d constitute an input waveguide bundle 4a, and eight optical waveguides including the four output optical waveguides 2a, 2b, 2c and 2d constitute an output waveguide bundle 4b. It is easily understood that the arrangement of FIG. 2A is topologically equal to that of FIG. 1.
These waveguide bundles 4a and 4b are silica-based single-mode optical waveguide arrays formed on a substrate 3 by a known combination of the frame-hydrolysis deposition and the reactive ion etching technique. Each of the switch elements S00-S33 disposed at each one of the sixteen positions is a so-called Mach-Zehnder interferometer type 2.times.2 optical switch as shown in FIG. 2B.
In FIG. 2B, two optical waveguides 61a-61b and 62a-62b are placed in close proximity at two positions to form two directional couplers 63a and 63b. The coupling ratio of the directional couplers is set at 50% at the wavelength of a light signal. The optical path lengths of the two optical waveguides 61a-61b and 62a-62b between the two directional couplers 63a and 63b are set at an identical length (a symmetrical state) when thermooptic phase shifters 64a and 64b, which are made of thin film heaters and are disposed over the two optical waveguides, are not operated (in the OFF state).
Assuming that the power coupling ratio of the directional couplers 63a and 63b is k, the power of an input light signal to one optical waveguide is P10, the powers of output light signals from the bar path and the cross path are P1 and P2, respectively, and the phase difference taking place between the two waveguides connecting the two directional couplers 63a and 63b is .DELTA..phi., the input and output switching characteristics of the Mach-Zehnder 2.times.2 optical switch element can be expressed by the following equations: EQU P.sub.1 /P.sub.10 =(1-2k).sup.2 cos.sup.2 (.DELTA..phi./2)+sin.sup.2 (.DELTA..phi./2) (1) EQU P.sub.2 /P.sub.10 =4k (1-k) cos.sup.2 (.DELTA..phi./2) (2)
When the coupling ratio k=1/2, that is, when the directional couplers 63a and 63b are a 3-dB coupler, the input-output characteristics are as follows: First, when the switch is in the OFF state where the thin film heaters 64a and 64b are not supplied with power, the phase difference .DELTA..phi. is zero, and hence, the light signal is transmitted through the cross path, that is, through the path 61a-62b or 62a-61b. On the other hand, when at least one of the phase shifters 64a and 64b is made ON by applying power to the thin film heater, the optical path length difference of 1/2 wavelength corresponding to .pi. radian is produced between the two waveguides connecting the directional couplers. The phase difference .DELTA..phi. of .pi. thus produced switches the optical switch element into the bar state so that the light signal is transmitted through the bar path 61a-61b or 62a-62b. In this way, switching between the cross/bar states of the optical switch element is achieved. The conventional waveguide-type optical matrix switch using the 2.times.2 optical switch elements with such an arrangement as a basic element, however, presents the following problem in a fabrication process:
Although the coupling ratio of the directional couplers 63a and 63b must be exactly 50% at the wavelength of the light signal so as to achieve an ideal operation of the conventional Mach-Zehnder interferometer type optical switch element of FIG. 2B, it is difficult to set the coupling ratio at exactly 50% because some errors are inevitably involved in a practical fabrication process of the optical waveguides. This is because directional couplers are a very structure-sensitive optical device, and hence, the coupling ratio is readily varied by a width of the waveguides, by the separation between the two waveguides, and by very small process errors of the relative refractive index difference between the core and cladding of the waveguides, or the like.
When the coupling ratio of the directional couplers deviates from 50%, the light signal is not transmitted in its entirety through the cross path 61a-62b or 62a-61b in the OFF state, but leaks out of the cross path and enters the bar path 61a-61b or 62a-62b. In other words, so-called crosstalk takes place. This is an important problem to be solved in the fabrication process of the waveguide-type optical matrix switch.
For example, when the coupling ratio deviates from 50% upward or downward by 5%, approximately 1% of the light signal power leaks to the bar path 61a-61b or 62a-62b in each optical switch element so that only 15 dB extinction ratio can be achieved in the 4.times.4 optical matrix switch as shown in FIG. 2A. This becomes more serious as the scale of the matrix increases. In an 8.times.8 optical matrix switch, for example, the crosstalk characteristics deteriorates, and the extinction ratio declines to approximately 11 dB.
In a practical fabrication process of the optical waveguides, errors of approximately .+-.5% commonly take place, and even errors on the order of .+-.10% are not rare in setting the coupling ratio of the directional couplers at 50%. Thus, the coupling ratio sensitivity of the waveguide-type optical matrix switch has been one of the most important problems in fabricating it with a high yield.